This invention relates to a transparent conducting film formed on a substrate such as glass, ceramic, etc., which film is used as a heating element for preventing surface clouding (moisture collecting) or frosting of the electrodes of display devices such as liquid crystal displays (LCD), electroluminescent displays (EL), etc., and the windowpanes of automobiles, aircraft, buildings, etc. Especially, the invention relates to a method for forming said type of patterned transparent conducting film.
Hitherto, electrode materials having high visible light transmittance have been used for the heat-generating resistance element for preventing surface clouding or frosting of the electrodes of display devices such as LCD, EL, etc., and the windowpanes of automobiles, aircraft, buildings, etc.
Tin oxide/antimony oxide system (ATO), indium oxide/tin oxide system (ITO), etc., are known as typical examples of the transparent conductive materials usable for forming said transparent conducting film. These metal oxides are capable of easily forming a coating film on a glass or ceramic substrate to provide a transparent conducting film.
The following techniques are known for forming a transparent conducting film:
(1) vacuum deposition; PA1 (2) sputtering PA1 (3) CVD; PA1 (4) printing; PA1 (5) coating (dip coating, spin coating and roll coating).
However, methods (1), (2) and (3) mentioned above, in which usually a pattern is formed by photolithography, have problems such an extra number of steps for pattern formation and generation of waste liquid in the course of etching. For solving these problems, it has been contrived to perform patterning by a lift-off method. Nevertheless, because of the structural complexity and high cost of the film forming apparatus, there has not yet been reached any satisfactory solution to the essential problems of production cost and productivity. On the other hand, methods (4) and (5), although having the possibility to resolve said problems in methods (1) to (3), had a problem in that it is difficult with these methods to form a film which can well stand practical use.
For example, concerning method (4), when screen printing is performed with a paste-like composition containing light-curing tin and indium salts of the organic acids having a functional group or a composition made of an inorganic indium salt, an inorganic tin salt, a non-aqueous silica sol and an organic solution of a cellulose compound, or when offset printing is conducted with a composition comprising an indium compound, a tin compound, a gum type resinous rosin and a terpene type high-boiling solvent, there arise typically the following two problems.
The first problem is that the film thickness at the `solid` portion tends to vary greatly while the film thickness at the pattern end becomes small, both of which are destined to occur in screen printing and offset printing. Another problem is that the cellulose compound and the resin such as gum type natural resinous rosin contained in the composition for obtaining good printability tend to remain in the form of free carbon in the film after thermal decomposition, making unavoidable the deterioration of the electrical, optical and mechanical properties of the film.
In the case of method (5), since no high-molecular weight compound able to produce free carbons is contained in the transparent conducting film forming composition, printing of a low-viscosity composition is hardly possible. Further, with spin or dip coating, it is difficult to perform direct pattern formation in the film forming process, and pinholes tend to form in the film due to insufficient cleaning or the presence of oil films.